1. Field of the Invention
The present invention relates generally to optical ring networks.
2. Related Art
A self-healing optical ring network has three or more ring elements (also called nodes) connected in a logical loop. Each ring element is connected to two other ring elements by working fiber and spare fibers (also called channels). When wavelength division multiplexing (WDM) is used a working channel and a spare channel can be carried on one or more fibers. A working channel carries traffic between ring elements during a normal mode of operation. A spare channel also carries traffic between ring elements, but a spare channel only does so when one of the working channels in the ring experiences a failure.
Ring elements can include an add/drop multiplexer (ADM). An ADM can pass traffic between the ring network and other equipment such as an electrical broadband digital crossconnect switch (DXC) and line terminal equipment.
In a typical opto-electronic Synchronous Optical Network (SONET) ring network, an ADM includes a failure detection unit that detects when a channel failure has occurred. For example, an ADM detection unit will detect a channel failure if it senses a loss of signal condition. In response to detecting a failure, the ADM sends a failure indication (also known as an alarm indication) to a central network management system, and the ADM switches traffic onto a spare channel using loopback to provide ring restoration.
FIG. 1A illustrates an example self-healing optical ring network having four ring elements 102, 104, 106, and 108. In normal mode, the working channels 110, 112, 114, and 116 carry data around the ring in a single direction and the spare channels 118, 120, 122, and 124 are idle. When a failure occurs in a ring configuration, the spare channels not affected by the failure are activated and route the traffic around the fault in the opposite direction.
FIG. 1B illustrates the operation of a self-healing optical ring when working channel 110, which is designed to carry traffic between ring element A and ring element B, experiences a failure. After ring element A detects a failure in working channel 110, ring element A switches traffic arriving on working channel 116 onto spare channel 124 in the opposite direction of the traffic flow on working channel 116. Similarly, after ring element B detects a failure in working channel 110, ring element B switches traffic arriving on spare channel 120 onto working channel 112 in the opposite direction of the traffic flow on spare channel 120. In this manner, the ring self-heals upon sensing a break in the ring.
While a present-day opto-electronic SONET ring design has the advantages of simplicity and fast switching speed, it has the drawback of an inefficient spare to working capacity ratio. The spare to working capacity ratio is the ratio of the number of spare channels to the number of working channels. In opto-electronic SONET ring networks the spare to working capacity ratio is 1:1. That is, for each working channel there must be a corresponding spare channel.
A self-healing optical network is needed that retains the speed and simplicity of a self-healing SONET ring network while providing more efficient use of spare channels.